An exposure apparatus conventionally used for transferring a pattern of circuit form applied on a reticle acting as a mask via an optical projecting system onto a photosensitive substrate (namely, a wafer or a glass plate coated with e.g. a photoresist and referred to as a wafer hereinafter) is of a reduced projection type exposure apparatus of step-and-repeat type (a stepper). More recently, a reduced projection exposure apparatus of step-and-scan type is employed for transferring a large-sized mask pattern to a wafer at higher definition without increasing the size of an optical projecting system.
Also, because the optical projecting system in such a type of the projection exposure apparatus is intended to increase the resolution and thus has a higher numerical aperture and hence a smaller depth-of-focus, it includes an auto-focusing (AF) system for setting (focusing) an imaging surface of the optical projecting system with the surface of the wafer within a permissive range of depth-of-focus of the projecting system and an auto-leveling (AL) system for adjusting the tilting of the exposure surface of the wafer. It is thus necessary in either the AF and the AL system to accurately measure the surface position at more than one point in the exposure area of the wafer along the optical axis of the optical projecting system.
Particularly, the AF system comprises a focusing point detecting system of oblique beam incident type for directing a beam of non-photosensitive illumination light to e.g. a photoresist at an angle to the optical axis of the projecting system and receiving its reflection to determine the level of defocusing of a projected image on the wafer surface (referred to as an AF sensor), and a stage system responsive to the result of measurement from the AF sensor for controlling the focusing location (along the optical axis in the optical projecting system) on the wafer. As an AF sensor of a conventional type, the following technologies are known.
Firstly, as disclosed in Japanese Patent Laid-open Publication (Hei)7-221013, a sensor of the first conventional type for detecting the position in two dimensions is provided in which a slit pattern of light is obliquely projected onto the effective area of a surface to be examined and re-imaged on a two-dimensional image pick-up device. More specifically, a two-dimensional change in the slit pattern is measured and detected as a displacement of the surface to be examined. For avoiding unwanted effects of an existing base pattern projected on the surface to be examined, the slit pattern is directed at an angle to the existing base pattern on the surface to be examined.
Secondly, as disclosed in Japanese Patent Laid-open Publication (Hei)5-129182 (U.S. Pat. No. 5,633,721), a sensor of the second conventional type is provided similarly in which a slit pattern of light is obliquely directed to the two dimensional area of a surface to be examined and re-imaged on a two-dimensional image pick-up device such as a CCD to measure a range (here, along the normal line) of the two-dimensional position of the surface to be examined from a lateral shift of each portion of the re-imaged pattern. As this type of the sensor allows the slit pattern of light to be obliquely projected to the surface to be examined, the imaging surface conjugate with the surface to be examined possibly has a dithered surface significantly tilted from the perpendicular to the optical axis. Accordingly, if the imaging surface is positioned parallel to the dithered surface, the luminous intensity of a projected image on the projection surface will be declined. For compensation, the second conventional sensor employs a diffraction grating or the like to change the dithered surface to a surface vertical to the optical axis.
Thirdly, as disclosed in Japanese Patent Laid-open Publication (Hei)6-97045, a sensor of the third conventional type, similar to the second conventional sensor, permits a slit pattern of light to be projected on the surface to be examined and then its projection to be re-imaged on a light receiver slit so that a portion of the projection passing through the receiver slit is measured as detected light. In order to detect a lateral displacement of the re-imaged projection, the re-imaged projection is oscillated by an oscillating mirror. Also, the third conventional sensor uses a prism for shifting the dithered surface to a surface vertical to the optical axis.
Fourthly, as disclosed in Japanese Patent Laid-open Publication (Hei)6-188172 (U.S. Pat. No. 5,191,200), a sensor of the fourth conventional type is provided in which for detecting the position of a surface to be examined at one single point in the center of an exposure field in the projecting system, a pattern of first diffraction grating is projected to the surface to be examined and its projection is then re-imaged on a second diffraction grating so that a portion of the re-imaged projection passing through the second grating is received. Accordingly, the fourth conventional sensor is substantially equivalent to the third conventional sensor, in which the slit pattern is replaced by a diffraction grating pattern. In the fourth conventional sensor, the reflection of light on the surface to be examined is received via a polarizer, a birefringent element (a Savart plate), a polarizing modulator, and an optical detector for detecting a lateral displacement of the image of a pattern resulting from variation in the surface to be examined.